1. Field of the Invention
This invention relates to a static switching system and method for transferring an electrical load from a first electric power supply to a second electric power supply and to retransfer the load back to the first electric power supply, all as dictated by operating conditions and also eliminating transients in such retransferring.
2. Description of the Prior Art
It may be explained here that in applications of uninterruptible power supplies (UPS) to larger critical loads, e.g., a digital computer, there is a requirement that the electrical power supplied to the load be maintained at predetermined specifications in spite of (1) load step transients of all types, (2) failures of the UPS, (3) overloads of all types and (4) branch faults in the load equipment.
A typical prior art switching system is illustrated in block diagram form in FIG. 1. Thyristor switching is shown, however, other switching devices, including electromechanical, can be and are often employed. In the system shown in FIG. 1, the critical loads are fed normally from the power conditioning equipment, which typically comprises a static inverter supplied from a continuously available source such as a battery and charger. Upon failure of the utility source, the energy stored in the battery is converted to regulated alternating current by the inverter, so that power continuity at the critical loads is maintained, with switching means S.sub.1 remaining closed and S.sub.2 open.
The transfer switch is used to supply the load from the utility during events such as:
A. loss of the inverter to an internal failure, or routine maintenance of other modules in the power conditioner chain; and PA1 B. need to use the large power capacity of the utility to start very high in rush loads or to clear load faults, both beyond the normal capability of the uninterruptible power conditioning equipment. PA1 A. oversizing the inverter or other power conditioning means so as to reduce the per-unit load step, so that the percent transient amplitude will also be reduced; PA1 B. using special inverter designs such as step-wave or pulse width modulation approaches which tend to reduce the internal impedances within the power conditioning equipment. This approach again only reduces the effect by a given amount; the transient on transfer still occurs; and, PA1 C. inserting special compensating controls which transmit an impulse to the inverter voltage regulator at precisely the right time and in the precise amount necessary to minimize the transient on load application. While special controls such as this can sometimes be "tuned" to produce significant transient improvement on retransfer, very careful trimming adjustments are necessary for each individual equipment and application, seriously impairing the practicality of this approach.
During utility operation, S.sub.1 is opened, and S.sub.2 closed. When operation from the inverter is again desired, the transfer switch poles are restored to their original state, S.sub.1 closed and S.sub.2 open.
A serious problem has heretofore occured at the instant of transfer from the utility to the inverter output, namely, the well known transient voltage dip which results when a sudden load step is applied to a power source, such as a static inverter, which contains inherent and unavoidable internal impedances with its transformers, filters, wiring, etc., FIG. 2 illustrates the typical transient voltage dip effect.
In the past, various means have been used in attempts to minimize this problem including:
Hence, for previous attempts to solve the transfer transient problem, the transient event illustrated in FIG. 2 still applies (to a greater or lesser degree, of course, depending on the success of approaches outlined above). Contrasting with prior art, the transfer switching system, in accordance with the present invention, has the capability of completely eliminating the transfer transient, even when applied in conjunction with conventional types of power conditioning equipment which may be inherently slow in control response and contain relatively high internal electrical impedances, and therefore be particularly prone to transients due to step loading were it not for the apparatus and method of the present invention.